The petroleum industry has long sought to find new methods for treating heavy crude oils, highly waxy crude oils, and other petroleum materials in an effort to meet the ever-increasing demand for petroleum feedstocks and improve the quality of available crude oils used in refinery processes.
In general, heavy crude oils have a low API gravity, high asphaltene content, low middle distillate yield, high sulfur content, high nitrogen content, and high metal content. These properties make it difficult to refine heavy crude oil by conventional refining processes to produce end petroleum products with specifications that meet strict government regulations.
Many petroleum refineries perform conventional hydroprocessing after distilling crude oil into various fractions. Each fraction is then hydroprocessed separately. Therefore, refineries must utilize complex unit operations to handle each fraction. Further, significant amounts of hydrogen and expensive catalysts are utilized in conventional hydrocracking and hydrotreating processes under severe reaction conditions to increase the yield from heavy crude oil towards more valuable middle distillates and to remove impurities such as sulfur, nitrogen, and metals.
Additionally, distillation and/or hydroprocessing of heavy crude oil feedstock produces large amounts of asphaltene and heavy hydrocarbons, which must be further cracked and hydrotreated to be utilized. Conventional hydrocracking and hydrotreating processes for asphaltenic and heavy fractions also require high capital investments and substantial processing.
Currently, large amounts of hydrogen are used to adjust the properties of fractions produced from conventional refining processes in order to meet required low molecular weight specifications for the end products; to remove impurities such as sulfur, nitrogen, and metal; and to increase the hydrogen-to-carbon ratio of the matrix. Hydrocracking and hydrotreating of asphaltenic and heavy fractions are examples of processes requiring large amounts of hydrogen, and these two processes result in the catalyst having a reduced life cycle.
Supercritical water has been utilized as a reaction medium for cracking of hydrocarbons with the addition of an external source of hydrogen. Water has a critical point at about 705° F. (374° C.) and about 22.1 MPa. Above these conditions, the phase boundary between liquid and gas for water disappears, with the resulting supercritical water exhibiting high solubility toward organic compounds and high miscibility with gases. Furthermore, supercritical water stabilizes radical species.
However, utilizing supercritical water, without the use of external hydrogen as the reaction media for cracking, has some disadvantages such as coke formation, which occurs during the upgrading of hydrocarbons in the supercritical water fluid. Although the amount of coke produced from upgrading hydrocarbon in this manner is less than that produced by conventional thermal coking processes, coking must be minimized to increase liquid yield and improve the overall stability of process operation.
It is known in the industry that coke formation occurs in cracking using supercritical water if there is only a limited availability of hydrogen and feed hydrocarbon has high aromaticity. Several proposals have been suggested to supply external hydrogen to a feed hydrocarbon treated with supercritical water fluid. For example, hydrogen gas can be added directly to the feed stream. Carbon monoxide can also be added directly to the feed stream to generate hydrogen through a water-gas-shift (WGS) reaction between carbon monoxide and water. Organic substances such as formic acid can also be added to the feed stream to generate hydrogen through a WGS reaction with carbon monoxide, which is produced from decomposition of added organic substances and water. Additionally, a small amount of oxygen can be included in the feed stream to allow for oxidation within the feed matrix for generating carbon monoxide. This carbon monoxide can then be used in a WGS reaction for producing hydrogen. However, feeding external gas and/or organic substances into a liquid stream increases costs and introduces added complexity to the process.
Highly waxy crude oil contains substantial quantities of paraffinic compounds that have elevated boiling points and considerable molecular weights. These properties result in high pour points and difficulties in the transferring capability of the crude oil through pipelines and oil tankers. Thus, highly waxy crude oil has come to be regarded as a non-conventional petroleum source. Furthermore, the highly waxy crude oil has a very low content of unsaturated hydrocarbons, which makes it unsuitable as a feedstock for most current refining processes and petrochemical processes. For example, to distill straight-run naphtha from highly waxy crude oil requires severe reforming treatment to increase aromatic and olefinic contents for improving octane rating of motor gasoline.
Upgrading of highly waxy crude oil is possible through conventional thermal or catalytic cracking, but such treatment produces substantial amounts of coke and consumes large amounts of hydrogen and catalyst. In addition to thermal and catalytic cracking, the problems caused by the high pour point of highly waxy crude oil can be reduced by solvent dewaxing and/or addition of pour point depressants. However, all of these methods suffer disadvantages.
As noted earlier, thermal coking produces large amounts of solid coke as a by-product, which is an indicator of the loss of valuable hydrocarbon feedstock. Catalytic hydrocracking requires large amounts of hydrogen and the regular replacement of spent catalyst. Solvent dewaxing requires a wax disposing system and a solvent recovery system, which adds to complexity and expense. Pour point depressants are expensive and change the end product in undesirable ways.
Therefore, it would be desirable to have an improved process for upgrading heavy and highly waxy crude oils with supercritical water fluid that requires neither an external supply of hydrogen nor the presence of an externally supplied catalyst. It would be advantageous to create a continuous process and apparatus that allows for the upgrade of the whole crude oil, rather than the individual fractions, to reach the desired qualities such that the refining process and various supporting facilities can be simplified. Furthermore, it would be desirable to have a process that could be implemented at the production site without the use of complex equipment. Additionally, it would be most desirable to make the process be one that is conducted in a continuous fashion.